2015ApJ...809...66V

Interstellar dust grains produce X-ray halos around bright sources due to small-angle X-ray scattering. Numerous studies have examined these halos, but no systematic study has yet tested the available halo data against the large number of well-defined dust models in circulation. We have therefore obtained the largest sample to date of X-ray dust halos from XMM-Newton and Chandra, and fitted them with 14 commonly used dust grain models, including comparisons with the optical extinction, A_V, where available in the literature. Our main conclusions are summarized as follows. (1) Comparing A_V with N_H values measured via X-ray spectral fits, we find a ratio of A_V/N_H (10²¹ cm^–2) = 0.48 ± 0.06, in agreement with previous work. (2) Out of 35 halos, 27 could be fit by one or more grain models, with the most successful models having maximum grain radius a_max 0.4µm and fewer large grains than the less successful models. This suggests that the diffuse ISM does not contain a signicant presence of grains with a_max 0.5µm. (3) Most halos were best fit assuming a single dust cloud dominated the scattering, rather than smoothly distributed dust along the sightline. (4) Eight sources could not be fit with the models considered here, most of which were along distant (d 5 kpc) sight lines through the Galactic thin disk. (5) Some sight lines had halos with observed X-ray scattering optical depth τ_sca/A_V that were signicantly different than expected. This may result from an inhomogeneous dust distribution across the halo extraction area.